Electronic components are frequently designed and build with a group of solder bumps or balls as the electrical contacts to be interconnected to a substrate or other components. Such an arrangement is advantageous for providing a relatively large number of connections in a small area which can be simultaneously joined in one solder reflow operation. One example of such an arrangement at the semiconductor level is the flip chip and controlled collapse chip connection (hereinafter referred to as "C4") as described in Seraphim, Lasky, and Li, "Principles of Electronic Packaging", McGraw Hill, 1989 pages 595-598 which is incorporated herein by reference. Another example at the chip carrier level is the ball grid array package such as is described in U.S. Pat. No. 5,572,405 by Wilson et al.
Various encapsulant materials have been developed and used to surround the individual solder connections after the electronic component is joined to a substrate or another component. The purpose of the encapsulant may be to protect the solder from exposure to detrimental environmental conditions such as moisture, dust, or corrosive atmospheres or even the oxygen in the surrounding air. Another purpose of the encapsulant, well known in the art is to strengthen the solder joints in order to make these more resistant to stresses and in particular to stresses caused by differing thermal expansion coefficients between the component and substrates which are interconnected by the solder joints.
The encapsulant can be applied as a liquid after the solder joints are formed by dispensing the encapsulant through a nozzle or needle in the region about the periphery of the joints. After dispensing, the liquid flows into the space between the component and substrate and around the connections by capillary action. This is a slow process and subject to defects caused by entrapped air packets, debris, or contamination from the solder joining e.g. reflow process. The encapsulant is typically cured by heating for 1-2 hours at about 130.degree.-160.degree. C. Dispensing and curing of an encapsulant is well known and described in many documents such as U.S. Pat. No. 5,661,088 by Tessier et al.
It is also possible to place a quantity of encapsulant material onto a substrate prior to component attach. For example, a flip chip is then placed on the substrate with solder bumps down to compress and pierce the encapsulant to form a compression bond to the substrate. The solder bumps are then reflowed to make solder connection to the substrate while simultaneously curing the encapsulant as described by McGovern et al. in Electronic Packaging & Production, February 1998 pp. 68-76.
A flip chip may also be electrically connected to a substrate by bonding the two together with an adhesive material initially provided in either a liquid or sheet form as described in U.S. Pat. No. 5,714,352 by Hogerton et al and also earlier by Tanaka in U.S. Pat. No. 5,363,277. Bonding elements on the flip chip are forced into contact with corresponding pads on a substrate and held in place by the adhesive without forming solder joints. If the adhesive is provided in sheet form, the bonding elements are forced through the sheet. It is important that the substrate be deformable so that a "wiping action" occurs between the chip bonding elements and the pads when the bonding elements are forced into contact with the pads. In addition the deformation accommodates variation in height of the bonding elements. Because the bonding element and pad eventually penetrate into the surface of the substrate a predetermined distance, this invention is limited to use on deformable substrates.
In the foregoing encapsulation methods, the encapsulant material is applied to the substrate either before or after a component with solder bumps is attached. It would be advantageous, however, to apply the encapsulant to the component prior to attachment thereby permitting the apply procedure to be performed by a component supplier as part of the component manufacturing process. U.S. patent application Ser. No. 08/939,302 filed Sep. 19, 1997 U.S. Pat. No. 5,888,850 and assigned to the same assignee uses an imersion process with components. In the case of flip chips, for example, applying the encapsulant while the chips are still in wafer form prior to dicing could lead to batch processing and the resulting economics of scale.
Substrates, especially circuit boards, supporting a plurality of components frequently have components of many differing sizes and shapes, often requiring a variety of encapsulation quantities and procedures. Having the encapsulation material already applied to a component prior to attachment to a substrate eliminates or reduces the need to accommodate and provide for this variety and is a desirable capability.
Therefore, in accordance with the teachings of the present invention, there is provided an encapsulation method which can be applied to electronic components. It is believed that such a method constitutes a significant advancement in the art.